Over the years, a full range of relay products from highly specialized relays for communication equipment to general-purpose relays have been designed to control nearly every function in commercial and industrial processes used in everything from household appliances to industrial machinery. A power generation plant is one example of an industrial process in which a large number of relays are used. The relays in a power generation plant can be used to control a wide variety of equipment such as motors, pumps, solenoids and lights. However, even the best relays may fail at some point.
In the example scenario above, a control system needs to monitor the relays within the power plant to determine their status in order to ensure that certain functions associated with the process are being performed. In particular, contact input status detection circuits are used to detect the status of relay contacts when in use in the field. The contact input status detection circuit monitors provide an indication of potentially degraded electrical relay performance due to contamination.
In industrial environments, contamination routinely interferes with the operation of the relay's contact. Contaminants, which can include oxidation films or foreign particles, tend to produce contact resistance readings that are either high or unstable. Contamination commonly happens with low current applications, usage in high temperature and humidity environments, and during extended periods of storage.
For example, in small currents and low voltage applications, oxidation of relay contact is simply a buildup of corrosion on relay contact surfaces over a period of time. The contacts develop oxidation, which is a thin layer of oxide on the contact surface. It causes problems by increasing the resistance across the contacts which, depending on the amplitude of the voltage being switched, can cause loss of signal or overheating of the contacts.
Oxidation on relay contacts is especially a problem with small currents and low voltages, because they cannot punch through the oxide layer once it accumulates and becomes too thick. However, higher voltages may punch through the oxidation layer during relay switching. Thus, the issue with switch contacts is voltage. Current through the contact is strictly a function of the voltage and the impedance of the circuit the contact completes. By varying the current, the voltage varies. Once the voltage is great enough, the punch through voltage is achieved.
One conventional approach to resolve this issue for small currents and low current applications is by passing the required wetting current through the relay contacts so that it punches through the oxide layer. The wetting current is the minimum current needed to flow through a contact to break through any film (contact oxidation) that may have been deposited on the switch. Typically, this has been performed by different group of boards connected in order to support a wide range of contact input voltages.